Thresh imo, one freaking hook in the back of your teammate and woop!, teamfights fucked. Or the wonderful lantern jungle ganks; see a jarvan flying towards you with a lantern, then instantly jumping on you with his combo.

Probably Lee imo, Does everything every jungler can do, ridiculously strong early game, snowballs like crazy, reveals stealth (ok, I'm just nitpicking this but it can make a difference when that low hp Twitch tries to stealth away), can build almost anything on him, sustain, AS slow. I could go on but you get the point.